This invention relates to single crystal CVD (chemical vapour deposition) diamond for use as a wear surface, and in particular for use in wire drawing dies.
In this application, a ‘wear surface’ is any surface of a solid which is subject to a sliding or moving contact with another surface or fluid, where that surface is particularly suited to such application because of its low wear rate, and optionally in addition because of one or more of the following properties: a) low displacement of the surface under the loads associated with the application, b) low frictional forces generated at the surface, or c) resistance of the surface to adverse modification, such as degradation of an optical surface.
A wire drawing die comprises a body of hard material having a hole formed therethrough and mounted in a suitable support. Such a body is known as a wire drawing die blank. Wire having a diameter greater than the diameter of the hole is drawn through the hole thereby reducing its diameter. The internal surface of the die thus forms a wear surface. A variety of hard materials may be used for the body. One of the best materials known is diamond because of its hardness and wear resistance. Diamond which has been used in wire drawing dies includes natural diamond and HPHT synthetic diamond.
Two parameters are considered to be key in the application of a wire die. First and foremost is the overall wear rate. Second is any ovality or out of round variation, which arises due to asymmetric wear. In single crystal diamond typically the asymmetry of the wear is reduced by using {111} plates with the wire forming hole aligned with the <111> direction parallel to the plate normal (i.e. the wire aligned perpendicular to the plate major {111} faces), although in the production of wires with cross-sections which are not round, other orientations of plate and hole direction may be preferable.
The wear rate of diamond is very sensitive to the conditions under which it is measured. As described hereinafter, this can be rationalized in terms of the range of possible wear mechanisms which are thought to operate and which may vary in their contribution according to the conditions of the test. However, no definitive model exists at this time. For a specific set of conditions, the minimum wear rate so far obtained in diamond (and therefore the most beneficial as a wear surface) is generally accepted to be a fundamental property of the material, typified by the behaviour observed in high quality single crystal natural diamond. Poor availability and the high cost of suitable natural diamond has led to the development of a substantial market in HPHT synthetic single crystal diamond. Interest has been expressed in using CVD single crystal diamond, but this has not been commercially available to date.
Recently it has been disclosed that high quality CVD diamond of high purity can be made with advantageous electronic properties [International Patent Publication WO 01/96634]. That disclosure shows that by controlling the incorporation of impurities and lattice defects such as dislocations, it is not only possible to achieve the electronic performance of natural diamond, but to substantially exceed natural diamond in certain key electronic parameters. This was not predicted but has been rationalized retrospectively in terms of the effect such impurities and defects have on trapping charge carriers.